Speed reducers such as cycloidal and harmonic drive speed reducers have been used extensively for robot joints. In modules incorporating such speed reducers, the joint bearing is occasionally integrated with the speed reducer. However, such speed reducers have not fully supported the particular needs of robot construction. For example, it is desirable that the robot joint be hollow to pass cables and service lines through the joints from a base to serially connected joints of the robot arm and finally to its end effector. It is also desirable to avoid the duplication of bearings at the joints wherein the robot joint, the speed reducer, and possibly a motor which drives the speed reducer, have their own independent bearings.
Robots are also subjected to occasional overloads resulting from interference with peripheral equipment. Such interferences could damage the robot and render it inoperable unless the robot joints and reducers have inherently high overload carrying capacities. Furthermore, the robot operation under tight servo controls demands a robot joint having minimal backlash and high stiffness. Robot construction also benefits from the integration of components to simplify manufacturing, assembly, and maintenance and reduce robot cost.
Some commercial speed reducers offer some of these advantages. For example, rotary vector reducers, known as RV reducers, and harmonic drives can be integrated into the robot joints and share common bearings. RV reducers also have low backlash and high overload carrying capacity and can be provided with a small center hole. However, the RV reducer is heavy in weight, costly, and does not offer a hollow center core large enough to be of appreciable value. Harmonic Drives have excessive backlash, low overload capacity, and, do not provide a hollow center. Both drives have very limiting motor mounting options. Custom reducers, utilizing gear trains, can be built to provide the required characteristics but at undesirably high costs, or, with excessive compliance.
U.S. Pat. Nos. 4,601,216 and 2,150,540 to Inoue et al and Chilton, respectively, disclose hollow reduction gears.
U.S. Pat. Nos. 3,705,522 and 3,596,538 to Ogawa and Braun, respectively, disclose speed change mechanisms.
Italian Patent Document No. 367144 and PCT Patent Document No. 8500859 also disclose various types of speed change mechanisms.
U.S. Pat. Nos. 4,918,344, 4,850,457, 4,690,010 and 3,239,699 to Chikamori et al, Taig, Matsumoto et al, and Ferrary, respectively, disclose combined electric motor and speed reducer mechanisms.
In general, it is desirable to have a speed reducer adapted for integration in a robotic rotary joint which has low backlash, and provides optional speed reducing ratios, high overload capability and a large hollow center. Such a rotary joint should be produced at a relatively low cost, have fewer components with high reliability and provide a compact package.